DescriptionThis frog is a member of the mountain yellow-legged frog complex which is comprised of two species: Rana muscosa and Rana sierrae. Both species are highly aquatic and are always found within a meter or two from the edge of water. Rana sierrae is yellowish or reddish brown from above, with black or brown spots or lichen-like markings. Toe tips are usually dusky. Underside of hind legs and sometimes entire belly is yellow or slightly orange, usually more opaque than in foothill yellow-legged frog, Rana boylii. Yellow often extends forward to level of forelimbs. Dorsolateral folds present but frequently indistinct. The tadpoles are black or dark brown and are large (total length often exceeds 10 cm) and metamorphose in 1-4 years depending on the elevation.
Rana sierrae differs from Rana muscosa in having relatively shorter legs. When a leg is folded against the body the tibio-tarsal joint typically falls short of the external nares. The mating call of R. sierrae is significantly different from that of R. muscosa in having transitions between pulsed and noted sounds. Both species call underwater. Males can be heard above water but only from a short distance away (<2 meters). The two species also differ in mitochondrial DNA. The mitochondrial DNA, male advertisement calls, and morphology datasets are geographically concordant (Vredenburg et al. 2007).

This montane species once occurred in California and Nevada, USA but is now extinct in the state of Nevada. Rana sierrae ranges from the Diamond Mountains northeast of the Sierra Nevada in Plumas County, California, south through the Sierra Nevada to the type locality, the southern-most locality at Matlock Lake just east of Kearsarge Pass (Inyo County, California). In the extreme northwest region of the Sierra Nevada, several populations occur just north of the Feather River, and to the east, there was a population on Mt. Rose, northeast of Lake Tahoe in Washoe County, Nevada, but, as mentioned above, it is now extinct. West of the Sierra Nevada crest, the southern part of the R. sierrae range is bordered by ridges that divide the Middle and South Fork of the Kings River, ranging from Mather Pass on the John Muir Trail east to the Monarch Divide. East of the Sierra Nevada crest, R. sierrae occurs in the Glass Mountains just south of Mono Lake (Mono County, CA) and along the east slope of the Sierra Nevada south to the type locality at Matlock Lake (Inyo County, CA).

Life History, Abundance, Activity, and Special BehaviorsSimilar to R. muscosa, breeding begins soon after ice-melt or early in spring and can range from April at lower elevations to June and July in higher elevations (Wright and Wright 1949; Stebbins 1951; Zweifel 1955). Eggs are deposited underwater in clusters attached to rocks, gravel, and under banks, or to vegetation in streams or lakes (Wright and Wright 1949; Stebbins 1951; Zweifel 1955). Livezey and Wright (1945) report an average of 233 eggs per mass(n=6, range 100-350). Eggs contain a vitelline capsule, and three gelatinous envelopes, all clear and transparent (see illustrations in: Stebbins 2003). In laboratory breeding experiments egg hatching times ranged from 18-21+ days at temperatures ranging from 5-13.5 °C (Zweifel 1955).
The length of the larval stage depends upon the elevation. At lower elevations where the summers are longer, tadpoles are able to grow to metamorphosis in a single season (Storer 1925). At higher elevations where the growing season can be as short as three months, tadpoles must overwinter at least once and may take 2 or 4 years of growth before they are large enough to transform (Wright and Wright 1949; Zweifel 1955).

Trends and ThreatsRana sierrae is critically endangered, along with its sister species Rana muscosa. These frogs have declined dramatically despite the fact that most of the habitat is protected in National Parks and National Forest lands. A study that compares recent surveys (1995-2005) to historical localities (1899-1994; specimens from the Museum of Vertebrate Zoology and the California Academy of Sciences) found that 92.5% of populations have gone extinct (11 remaining out of 146 sites; Vredenburg et al. 2007).

The two most important factors leading to declines in R. sierrae and R. muscosa are disease and introduced predators.

Introduced trout prey on R. sierrae (Needham and Vestal 1938; Mullally and Cunningham 1956)and have been implicated in a number of studies as one of the sources of decline (Bradford 1989; Bradford et al. 1993; Jennings 1994; Knapp 1996; Drost and Fellers 1996; Knapp and Matthews 2000). In fact, as early as 1915 Joseph Grinnell and his field crews (Grinnell and Storer 1924) noticed that Rana sierrae rarely survived in lakes where trout were planted. Whole lake field experiments have shown that when non-native trout are removed, both Rana sierrae and Rana muscosa populations rebound (Vredenburg, 2004; Knapp et al. 2007).
While it is clear that introduced trout negatively affect R. sierrae and R. muscosa mainly through predation on tadpoles, trout also compete for resources with adult frogs. A food web study that used stable isotopes to trace energy through the Sierran lake food webs concluded that introduced trout are superior competitors and suppress the availability of large aquatic insects that make up a major portion of the diets of adult frogs (Finlay and Vredenburg 2007).

A lethal disease, chytridiomycosis, caused by an aquatic fungal pathogen Batrachochytrium dendrobatidis, or Bd (Berger et al. 1998) has caused population extinctions in R. muscosa and R. sierrae in the Sierra Nevada (Rachowicz et al. 2006). Long-term studies reveal that infection intensity is key; once a critical threshold of Bd fungal infection is reached, death ensues (Vredenburg et al. 2010). Population extirpation is the most common outcome, but a few mountain yellow-legged frog (Rana sierrae and Rana muscosa) populations have survived in low numbers. Modeling shows that chytriodiomycosis outcome at the population level (extirpation vs. persistence) can result solely from density-dependent host-pathogen dynamics, which may hold for other wildlife diseases as well (Briggs et al. 2010). In an effort to rescue the last surviving frogs, the Vredenburg lab is treating adult Rana sierrae in the field with anti-fungal medication; frogs are bathed for five minutes daily over the course of a week (Lubick 2010).

Other possible causes for decline in R. sierrae include air pollution (pesticide drift; Davidson et al. 2002; Davidson 2004), UV-B radiation, and long term changes in weather patterns, especially concerning the severity and duration of droughts. Acidification from atmospheric deposition has been suggested as another cause, but Bradford et al. (1994) found no evidence to support this hypothesis.

Relation to HumansMountain yellow-legged frogs (the amphibian species complex including both Rana muscosa and Rana sierrae) were once the most common vertebrates in the high elevation Sierra Nevada. Documented historical accounts go back to the turn of the last century (1915) from surveys conducted by Joseph Grinnell and Tracy Storer (published in 1924) from the University of California's Museum of Vertebrate Zoology. Joseph Grinnell was instrumental in the foundation of Yosemite National Park, one of the jewels of the American National Park Service.